Thermal Resistance Calculator
Thermal resistance describes how well a material or assembly resists the flow of heat, measured in degrees Celsius per watt (°C/W). Lower thermal resistance means better heat transfer. In electronics, the thermal path from a chip to ambient air is a series chain of resistances: junction-to-case, case-to-sink (thermal paste or pad), and sink-to-ambient (the heat sink itself). This calculator handles both the heat sink chain and single material layers.
The thermal model is an electrical analogy: heat flow (watts) is like current, temperature difference is like voltage, and thermal resistance is like electrical resistance. Just like Ohm's law:
Delta-T = P x R_thermal
And for series resistances in a heat sink assembly:
R_total = R_junction-case + R_case-sink + R_sink-ambient
For a material layer (conduction through a slab):
R = thickness / (k x A)
Where k is thermal conductivity in W/(m·K) and A is the cross-section area.
Typical thermal conductivities:
- Copper: 385 W/(m·K)
- Aluminum: 200 W/(m·K)
- Steel: 50 W/(m·K)
- Thermal paste: 1-10 W/(m·K)
- FR4 PCB: 0.3 W/(m·K)
- Air (still): 0.026 W/(m·K)
For electronics design, the junction temperature must stay below the component's rated maximum (often 125°C or 150°C for silicon). If your calculated junction temperature exceeds this limit, you need a better heat sink, forced airflow, or a lower-power design.